I'm not sure if you have a hard top or convertable but I know for the convertables you have to grab your signal for your inline converter off the rear mids, use the black with green for the positive and the black with white for the negative. You probaly already have a inline converter which allows you to convert you signal into rca outputs but if you don't you'll need one. If you car is a hard top then you need to splice into the data cable which is located under the rear deck lid. Its a small black or grey box with a few wires going into it. Sorry for the lack of information on the hard top I have never hook up an amp on one but i know it's easier than the convertables. Try this link out I know the pic's arn't very good but it may help.
http://www.mustangworld.com/ourpics/fcar/despeeds1.htm